The present invention relates to an image forming apparatus for forming an image by transferring a color medium from a transfer medium to a sheet of paper in accordance with an image pattern.
Amont the known color image forming apparatuses of this type is a thermal head printer in which color media are melted by heat in accordance with an image pattern and then are transferred to a sheet of paper.
Small, low-priced, noise-free and capable of printing on ordinary paper, thermal head printers have recently been used as output devices of computers and word processors, and in copying apparatuses.
The thermal head printer uses a ribbon 2 shown in FIG. 1. Ribbon 2 has a yellow region 4, a magenta region 6, a cyan region 8 and a black region 10. These ink regions have the same width w and the same length l. To print a color image on a sheet of paper, first the yellow ink is transferred, then the magenta ink, the cyan ink and finally the black ink.
To print color images on sheets of different sizes, each ink region of ribbon 2 must be of the largest size. When color images are printed on sheets of a smaller size, that portion of each ink region which extends beyond the sheets is left unused. When ribbon 2 is large enough to print images on sheets of legal size (215.9 mm.times.355.6 mm) and images are printed on only sheets of letter size (215.9 mm.times.279.4 mm), the portion of each ink region, 214.9 mm long and 76.2 mm wide, is left unused. This is undesirable from an economical point of view.